In facilities for processing, storing and distributing gaseous products there are numerous relief valves for monitoring, controlling and relieving pressure build-up of gas flowing in duct systems. From time to time such relief valves need to be inspected and removed and replaced. Removal of a valve may expose the system to inconvenient or dangerous leakage of gas from the duct upstream of the removed valve particularly where a blocking valve does not achieve 100% shut-off. A common but usually inconvenient and expensive solution to this leakage problem is to shut down the parent system or at least to shut down the subsystem in the immediate vicinity of the relief valve to be removed. In a large gas treatment or production plant there are a great many such relief valves that need periodic attention as described above, or need removal in emergency situations, costing substantial downtime and expense. The present invention provides a solution that is reliable, substantially less expensive and more practical than conventional practice in gas collection, compression, treatment, distribution and/or storage facilities.